The invention concerns a method of detecting one or more free (or unoccupied) channels in an optical time-division multiplex, a device for implementing the method, and the use of said device.
The invention applies to communication networks using time-division multiplexing of encoded optical pulse streams (Optical Time Division Multiplex (OTOM)), referred to interchangeably hereinafter as an optical time-division multiplex or a time-division multiplexed optical signal.
The switching nodes of communications networks extract and insert information on the incoming multi-plexes in accordance with the routing intended for the information conveyed by the multiplexes.
In the case of time-division multiplex communications networks, in particular networks defined by the SDH (Synchronous Digital Hierarchy) standard, the information is in the form of frames made up of short pulses having a bit rate B (for example 10 Gbit/s) interdigitated bit by bit N times to obtain a resultant signal having a bit rate of Nxc2x7B (40 Gbit/s to 100 Gbit/s).
When the resulting signal reaches a switching node one or more frames of the signal can be physically extracted from the multiplex by an extraction operation (DROP function), leaving free time slots or channels in the resulting time-division multiplexed signal.
The same switching nodes can also insert information (ADD function) in unoccupied or freed channels of the incoming optical multiplex.
Until now ADD functions have been implemented electronically. To this end all the multiplexed signals have been detected and then stored in buffers so that the frames forming the signals can be reconstituted after processing.
Supervisory logic circuits have re-assigned frames after the DROP and ADD operations.
In the case of optical time-division multiplexes (OTOM) there is no prior art technique for implementing the above functions directly in the optical domain.
One compromise solution demultiplexes all of the optical signals forming the multiplex onto N channels, processes the channels electronically at the transmission or baseband frequency and then reconstitutes an optical time-division multiplexed signal by regenerating each of the basic optical signals by interdigitating them temporally.
The above solution is unsatisfactory because it involves changing from the optical domain to the electronic domain which requires complex and costly drop and add electronic functions in the case of processing bit rates in excess of 10 Gbit/s. For bit rates of 40 Gbit/s the electronic data storage and switching circuits operate at these bit rates directly.
In the optical time-division multiplex situation the above bit rate is achieved with four basic signals at 10 Gbit/s interdigitated to form the 40 Gbit/s multiplex and the data could be processed electronically by 10 Gbit/s circuits. However, this would require the OTDM signal to be demultiplexed and each signal from the multiplex to be processed electronically and independently and reconstituted afterwards.
The present invention solves the above problem by providing a solution that retains the signal in the optical domain during processing in the switching node.
To this end the invention proposes to detect free (or unoccupied) channels of the multiplex. It also proposes to identify the order of each free channel detected. It can also insert the encoded pulses of a new signal into the first free channel that has been detected and identified.
In particular, the invention proposes a method of detecting one or more free channels in a time-division multiplexed optical signal, the method comprising the following steps:
sampling successive optical pulses from at least one of said channels,
measuring the average optical power of the sampled pulses during a predetermined time period, and
generating a detection signal if said average optical power is less than or equal to a predetermined threshold value.
According to another feature of the invention the method comprises a step of identifying the order of the channels of the multiplex.
The identification step comprises the following steps:
dividing the energy of the multiplexed optical signal to form the same number of optical signals from that signal as there are channels forming the multiplex, and
carrying out the sampling, average power measurement and detection signal generation steps for each of said signals.
Advantageously the step of sampling the divided signals is performed using clock signals corresponding to the baseband frequency of the multiplexed optical signals offset relative to each other by a time period corresponding to the width of a channel.
The clock signals are obtained by recovering the baseband frequency B from the optical signals forming the multiplex.
According to another feature of the invention the method further comprises a step of inserting optical pulses of a new signal into the detected free channel.
The insertion step consists in injecting the successive optical pulses of the new signal into the first detected and identified free channel of the multiplex.
If other available channels of the multiplex have been detected, the insertion step consists in repeating all steps of the method as many times as there are free channels for the multiplex.
The invention further concerns a device for detecting and identifying one or more free channels in a time-division multiplexed optical signal, the device comprising:
a coupler having one input and N outputs for dividing the signal from the optical multiplex into the same number of signals as the number of channels forming the multiplex,
a circuit for sampling each output signal from the coupler clocked by a respective clock signal corresponding to the baseband frequency of the multiplexed optical signals and offset relative to each other by a time period corresponding to the width of a channel,
a low-frequency detector circuit adapted to measure the average power of the optical pulses during a particular time period of each channel identified by the sampling circuit, and
a circuit for generating a signal indicating detection of a free channel if the average power of the optical pulses is less than or equal to a particular threshold.
To this end, the sampling circuit comprises an optical gate on the path of each output signal of the coupler and the detector circuit and a delay line associated with each optical gate to obtain a time-delay between clock signals having a duration corresponding to the width of a channel.
The low-frequency detector and signal generator circuits are constituted by an array of photodiodes each followed by a repeater.
The device further comprises a device for recovering the clock at the baseband frequency B from the multi-plexed optical signals.
The invention also concerns an application of the detector device. This application consists in a system for inserting encoded optical pulses into a time-division multiplexed optical signal. The system comprises at least one insertion module including said detector device.
If the system includes a plurality of insertion modules, the modules are coupled to the line in cascade and the clock recovery circuit is the same for all the modules.
The system advantageously includes a single optical pulse generator source for all the insertion modules.
An insertion module further comprises a logic unit with N inputs and N outputs, each input being connected to an output of the detection signal generator circuit and each output being connected to an optical gate, opening of which is commanded by said output signal, said gates being connected to an optical coupler via a separate optical delay line imposing a relative delay at the output having a duration equal to the width of a channel, said optical gates receiving at another input the encoded optical pulses to be inserted in the first free channel detected.
The logic unit comprises AND gates and NAND gates, or their equivalent, adapted to deliver a signal to command opening of optical gates for the output corresponding to the first free channel detected.
The optical gates are constituted by semiconductor optical amplifiers (SOA) or doped fiber amplifiers (EDFA)